Wireless communication has become an integral part of modern life in personal and professional realms. It is used for voice, data, and other types of communication. Wireless communication is also used in military and emergency response applications. Communications that are made wirelessly rely on the electromagnetic spectrum as the carrier medium. Unfortunately, the electromagnetic spectrum is a limited resource.
Although the electromagnetic spectrum spans a wide range of frequencies, only certain frequency bands are applicable for certain uses due to their physical nature and/or due to governmental restrictions. Moreover, the use of the electromagnetic spectrum for wireless communications is so pervasive that many frequency bands are already over-crowded. This crowding may cause interference between and among different wireless communication systems.
Such interference jeopardizes successful transmission and reception of wireless communications that are important to many different aspects of modern society. Wireless communication interference can necessitate retransmissions, cause the use of ever greater power outlays, or even completely prevent some wireless communications. Consequently, there is a need to wirelessly communicate in the presence of electromagnetic interference that may otherwise hinder the successful communication of information. Use of horizontal polarization may improve communications reliability by reducing interference from predominantly vertically polarized signals in overlapping and adjacent frequency bands. Conversely the application of vertical polarization in an environment dominated by horizontally polarized interference may improve communication reliability.
Multipath fading results in reduced communications reliability, particularly where mobile devices pass through signal fades. Linearly polarized communication systems may generally be more susceptible to multipath fading than elliptically or circularly polarized systems. Mobile systems typically require an omni-directional antenna pattern on the client devices. An omni-directional antenna is characterized by an azimuthal radiation pattern that exhibits minimal antenna gain variation. Dual polarized (D-pol) omni-directional antennas allow for an increase in data throughput by exploiting nominally orthogonal vertical and horizontal polarizations associated with individual respective vertical and horizontal channels.
However, due to the nature of systems having D-pol omni-directional mobile antennas, the relative orientation of the vertical and horizontal polarizations between transmit and receive antennas may vary based on movement within the mobile system, or other inherent sources of transmit-receive antenna polarization misalignment. Additionally, the relative orientation of the vertical and horizontal polarizations in some antennas may be modified electronically, such as in circularly and elliptically polarized antenna systems.
For purposes of this disclosure, non-equal polarization is defined by two or more polarization states separated from each other on the Poincaré Sphere. In contrast, exactly orthogonal polarization is defined by two polarization states separated exactly by 180 degrees on the Poincaré Sphere. Additionally, nominally orthogonal polarization is defined by two or more polarization states that may deviate from being exactly orthogonal based on standard commercial manufacturing and deployment variations or tolerances.
There are a number of existing methods that address polarization mismatch between a transmitter and a receiver as well as multipath fading. For example, spatial diversity uses two or more antennas separated in space, thereby experiencing differing fading environments. Polarization diversity uses two or more antennas exhibiting differing polarization states. These two diversity techniques can take on various implementations. For example, the technique referred to as switched diversity, selects one of the antennas that exhibits the best quality metric. Maximum Ratio Combining (MRC) combines the outputs of all antennas simultaneously to maximize the Signal to Noise Ratio SNR. Minimum Mean Square Error (MMSE) and Optimal combining, like MRC, makes use of one or more antenna and can maximize SINR. However, MMSE and Optimal combining require carrier recovery as an integral component of the algorithm.